This is working by design as deleting a pledge write-off, in effect, removes/undoes the write-off's modifications to the pledge and the general ledger. If you would like the general ledger to reflect the adjustment date as the day the write-off was deleted, you can manually edit the post file (generated by the CRM Post to GL Process), to change the adjustment date to the desired date before adding/importing it to the general ledger outside of CRM.